In semiconductor technologies, masks (photomasks or reticles) are formed with predesigned integrated circuit (IC) patterns. The masks are used to transfer those predesigned IC patterns to semiconductor wafers in lithography processes. The masks may sometimes include one or more defects, due to multiple reasons. Any defect on a mask will be transferred to multiple semiconductor wafers and cause yield issues and quality concerns.
One source of mask defects is mask haze which may be introduced during mask making, handling, or lithography processes. For example, chemicals, such as SO2 and/or NH3, may be outgassed during mask making process. Despite some cleaning procedures, these chemicals continue to exist at the surface of the masks during deep ultraviolet (DUV) exposure process. The DUV exposure process generates highly energized photons that facilitate reaction of these chemicals with atmospheric gases to produce ammonium sulfate nano-crystals, causing mask haze. Mask haze may cause printing errors on a wafer, and thus should be eliminated.